Lock mandrels useful for releasably locking other well tools such as a wireline-retrievable SCSSV inside a flow conductor are well known. Such lock mandrels have previously been disclosed, for example, in U.S. Pat. Nos. 4,545,434 and 4,745,974. These patents teach the use of a running tool in a wireline tool string for driving a lock mandrel having a SCSSV connected to it into a landing nipple disposed in the flow conductor of a well. The safety valve and lock mandrel are driven downwardly into the landing nipple until a fixed no-go ring on the outer surface of the lock mandrel contacts an opposing no-go shoulder in the landing nipple. The running tool is not releasable from the lock mandrel until the SCSSV has been pressured open, permitting the running tool core to drop, and until the locking keys have engaged in the locking annulus of the landing nipple. The locking keys in the lock mandrel are engaged by jarring upwardly on the running tool.
Although the locking systems disclosed in U.S. Pat. Nos. 4,545,434 and 4,745,974 possess significant advantages when compared to the conventional locking systems previously known, problems have been encountered in using those locking systems when the wellbore is staggered or graduated between the landing nipple and that section of the bore in which the SCSSV is set. In such situations, the control line pressure exerted to pump the safety valve open during the locking process instead forces the lock mandrel and valve assembly upward before the lock mandrel locking keys can be set due to the area differential in the two sections of the staggered bore.
To avoid the foregoing problem, specially modified running tools, safety valves and landing nipples have been required to utilize the locking systems disclosed in U.S. Pat. Nos. 4,545,434 and 4,745,974 in staggered well-bores. These modifications permit the running tool to temporarily lock the lock mandrel in the landing nipple while the safety valve is being pressured open prior to completion of the locking procedure and removal of the running tool.
A releasable well tool locking system is therefore needed that can be used in a flow conductor having a staggered internal bore without the need for a specially modified running tool, safety valve or landing nipple. Such a locking system will preferably comprise a lock mandrel adapted to pre-prop the lock mandrel locking keys in the annular receptacle of the landing nipple to prevent the lock mandrel from being forced upward and out of alignment with the annular receptacle of the landing nipple while the safety valve is being pressured open. A preferred well tool locking system for use in staggered bores will desirably achieve the foregoing objectives in a structure that is adapted to be locked by a sleeve moving in the flow direction; that will not release the running tool unless the safety valve is functional and securely locked by the lock mandrel in the landing nipple; and that does not require the use of shear pins to release and remove the lock mandrel and safety valve from the well-bore.